In conventional sintering of multiphase tungsten alloys, the metals are mixed as powders, pressed, and sintered in liquid phase. With tungsten alloys this is done at temperatures higher than 1450.degree. C. Within the liquid phase at least three things must occur:
(1) formation of alloy; PA1 (2) coating of the tungsten granules; and PA1 (3) densification of the pressed body.
The necessarily long stay in the liquid phase results in strong granule growth, which results in strength decrease.
From U.S. Pat. No. 4,498,395, incorporated herein by reference, are known tungsten alloy powders which are already pre-alloyed, i.e., the tungsten grains are already coated with the binder phase. Pressed bodies of this powder are compacted by solid phase sintering, and the sintered parts are characterized by a polygonal structure of the tungsten phase. The structure is considerably finer than the ones of conventional tungsten heavy metal compositions which are prepared from the individual powders (W, Ni, Fe) by mixing, pressing, and sintering in liquid phase. The polygonal structure of the tungsten particles in compositions prepared according to U.S. Pat. No. 4,498,395 shows, however, a high contiguity of the tungsten phase, which means that there is a multitude of tungsten-tungsten grain boundaries. This situation can negatively effect the mechanical properties of the sintered tungsten heavy metals. There is impairment of the tensile strength and elongation at break especially if the alloy contains interstitial impurities such as oxygen, phosphorus, or sulfur and/or other components which are insoluble in tungsten. These impurities separate off at the tungsten grain boundaries and cause the grain boundary brittleness typical of tungsten.